I have a 97 Mirage. (1500) On the highway, and high in the rev range, it runs as well as it ever has, even after its 310,000kms. Lately at idle it is very rough. Even once it warms up, there is still a misfire at idle. There are no code faults recorded, and every time I have taken it in to someone they say there it nothing wrong with it. But there is. Sounds like an air/fuel mixture problem. On a cold morning it sounds like a steam engine trying to start, and if it stalls, it has trouble restarting (like it has a flat battery - the battery light comes on and all- but the battery is less than 6mths old) It also loses power if you go back a gear up steep hills. I have replaced the distributer cap, coil, rotor button, plugs, even the throttle body and oxygen sensor have been replaced. New exhaust, original cat. I replaced the gearbox 40,000km ago, could something of happened when that was getting done? The plug leads were replaced about 50,000km ago. The car has been regularly serviced its whole life and has never overheated. Any advice would be greatly appreciated.

At 310,000 km those 3 little valves per cylinder probably need some attention.
Sounds like a classic case of burnt valves or carbon buildup causing poor sealing.
Do a compression test on it.

I'm surprised these engines can go this distance, to be honest.

I would start with a comp test, most of these cars I get in with that sort of mileage do have low compression for the 1500 motors, other thing to look into is dirty injectors, again at that sort of mileage they may never have been cleaned

Thanks. Will get a compression tester. I had the injectors cleaned at its 180k service, but will check them again too. Thanks